With high-speed wireless services increasingly in demand, there is a need for more throughput per bandwidth to accommodate more subscribers with higher data rates while retaining a guaranteed quality of service (QoS). In point-to-point communications, the achievable data rate between a transmitter and a receiver is constrained by the available bandwidth, propagation channel conditions, as well as the noise-plus-interference levels at the receiver. For wireless networks where a base-station communicates with multiple subscribers, the network capacity also depends on the way the spectral resource is partitioned and the channel conditions and noise-plus-interference levels of all subscribers. In current state-of-the-art, multiple-access protocols, e.g., time-division multiple access (TDMA), frequency-division multiple-access (FDMA), code-division multiple-access (CDMA), are used to distribute the available spectrum among subscribers according to subscribers' data rate requirements. Other critical limiting factors, such as the channel fading conditions, interference levels, and QoS requirements, are ignored in general.
Recently, there is an increasing interest in orthogonal frequency-division multiplexing (OFDM) based frequency division multiple access (OFDMA) wireless networks. One of the biggest advantages of an OFDM modem is the ability to allocate power and rate optimally among narrowband sub-carriers. From a theoretical standpoint, OFDM was known to closely approximate the “water-filling” solutions of information theory that are capacity achieving. Some early work of Hirosaki, “An Orthogonally Multiplexed QAM System Using the Discrete Fourier Transform,” IEEE Trans. Communications, vol. 29, July 1981, pp. 982–989, based on an FFT implementation of OFDM achieved complexity and decoded bit count that was comparable to single-carrier counterparts. This inherent potential of OFDM achieved fruition in the design of discrete multi-tone systems (DMT) for xDSL/ADSL applications pioneered by J. Cioffi et al., “A discrete multi-tone transceiver system for HDSL applications,” IEEE Journal on Selected Areas in Communications, vol. 9, no. 6 Aug. 1991, pp 909–91.
OFDMA allows for multi-access capability to serve increasing number of subscribers. In OFDMA, one or a cluster OFDM sub-carriers defines a “traffic channel”, and different subscribers access to the base-station simultaneously by using different traffic channels. For more information, see Cheng and Verdu, “Gaussian multiaccess channels with ISI: Capacity region and multiuser water-filling,” IEEE Trans. Info. Theory, Vol. 39(3), pp 773—785, May 1993; Tse and Hanly, “Multiaccess fading channels—part I: Polymatriod structure, optimal resource allocation and throughput capacities,” IEEE Trans. Info. Theory, Vol. 44(7), pp 2796–2815, November 1998; and Wong et al., “Multiuser OFDM with adaptive subcarrier, bit and power allocation,” IEEE J. Select. Areas Commun., Vol. 17(10), pp 1747–1758, October 1999. These references indicate that there is a problem for multi-user communications and show that a full extent of centralized resource allocation in the context of OFDMA can substantially increase the capacity of a wireless network.
Existing approaches for wireless traffic channel assignment are subscriber-initiated and single-subscriber (point-to-point) in nature. Since the total throughput of a multiple-access network depends on the channel fading profiles, noise-plus-interference levels, and in the case of spatially separately transceivers, the spatial channel characteristics, of all active subscribers, distributed or subscriber-based channel loading approaches as fundamentally sub-optimum. Furthermore, subscriber-initiated loading algorithms are problematic when multiple transceivers are employed as the base-station, since the signal-to-noise-plus-interference ratio (SINR) measured based on an omni-directional sounding signal does not reveal the actual quality of a particular traffic channel with spatial processing gain. In other words, a “bad” traffic channel measured at the subscriber based on the omni-directional sounding signal may very well be a “good” channel with proper spatial beamforming from the base-station. For these two reasons, innovative information exchange mechanisms and channel assignment and loading protocols that account for the (spatial) channel conditions of all accessing subscribers, as well as their QoS requirements, are highly desirable. Such “spatial-channel-and-QoS-aware” allocation schemes can considerably increase the spectral efficiency and hence data throughput in a given bandwidth. Thus, distributed approaches, i.e., subscriber-initiated assignment are thus fundamentally sub-optimum.